They knew it in high school, when he swaggered into the student body presidency without having to campaign. They knew it in business, when locals could not turn their heads on many downtown streets for a time without encountering a building he owned.

They knew it when he was mayor — more iconoclast technocrat than conservative crusader — with a habit of scooping up sidewalk trash, calling in potholes and staring up, nodding, at park-dwelling homeless men with grievances to air.

And they know it now, on the occasion of a higher-profile confrontation: Mr. Corker, Tennessee’s junior Republican senator, has become President Trump’s chief congressional antagonist, warning of gross executive incompetence and a possible “World War III” in the most sweeping intraparty indictment yet of Mr. Trump’s fitness for office.

The nation has seen a reliable Republican from a deep-red state stunningly undermining his president. Chattanooga has seen it coming, for better or worse.

“He’s certainly not a grandstander, has never been that,” said Jon Kinsey, a Democrat, who preceded Mr. Corker as mayor. “He is simply saying what he thinks is the truth. That can be alarming.”

On paper, Mr. Corker — a drawling 65-year-old deficit hawk who detests the Affordable Care Act and declined to oppose a single Trump cabinet nominee — might seem an unlikely candidate to send up warning signals about a Republican president, particularly one he has often supported behind the scenes.

But in recent days, after saying he will not seek re-election next year, Mr. Corker has likened the White House to an “adult day care center” whose twittering inhabitant should “concern anyone who cares about our nation.” Mr. Trump has responded by mocking the 5-foot-7 Mr. Corker as “liddle,” misrepresenting his Senate record and inventing a story line that Mr. Corker had been duped into a tape-recorded interview with The New York Times.

Mr. Trump also said that Mr. Corker had “begged” for his endorsement (“I said ‘NO’ and he dropped out,” the president reported via Twitter). Mr. Corker has flatly denied this account, saying Mr. Trump had encouraged him to run and pledged his full support.

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In audio excerpts from an interview with Jonathan Martin, a New York Times reporter, Senator Bob Corker, Republican of Tennessee, spoke about President Trump's tweets and what Mr. Trump’s twitter feed means for diplomacy.Published OnOct. 9, 2017CreditImage by Tom Brenner/The New York Times

Those who know Mr. Corker best have viewed his unburdening as largely inevitable, the breaking point after a year of often pointed Trump critiques. Mr. Corker spoke out despite being one of the few establishment-aligned Republicans to develop a relationship with the president, who once weighed choosing him as secretary of state.

For Mr. Corker, whose surest path to re-election would most likely have involved hugging the president tight, the decision not to run has removed any electoral constraint against full candor. The choice also spares him the potential threat of a difficult primary challenge from his right.

All year, Mr. Corker has strained to maintain a careful balance. As chairman of the Foreign Relations Committee, he has forged close relationships with administration officials like Rex W. Tillerson, the secretary of state. And since last year, he and Mr. Trump had appeared to achieve something of a mutual respect themselves — the president finding in Mr. Corker another fortune-making builder and self-styled dealmaker in construction and real estate, who thrilled as much at the chase as at the profit.

“Do you think if we gave it back to them,” Mr. Corker once asked an associate, Michael Compton, moments after buying two of the largest real estate companies in town, “they’d let us try to buy it again?”

Before their latest flare-up, Mr. Corker and Mr. Trump were on semiregular speaking terms, golfing together this year with the retired N.F.L. quarterback Peyton Manning. “He is not like people think that he is,” Mr. Corker said of Mr. Trump at one point during the campaign, praising him as “courteous, kind, respectful.”

Yet after wrestling with how thoroughly to lash a president whose temperament he had come to doubt (he did, during the spring, suggest that the White House was in a “downward spiral”), Mr. Corker has turned to total bluntness, the logical extension of a professional life spent in near-permanent states of restlessness and cost-benefit analysis.

The evening before he said he would not seek re-election, Mr. Corker held a fund-raiser near the Capitol. One by one, guests raised questions about the Republican agenda, and what could reasonably be accomplished in Congress. Repeal-and-replace? Entitlement reform?

Not happening, Mr. Corker said on a loop.

He did not linger at the event. The next morning, he stepped into his Senate office with news.

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Mr. Corker ran for the Senate in 2006 after Senator Bill Frist announced his retirement. Mr. Corker’s race against a Democratic congressman, Harold E. Ford Jr., was one of the most watched of the year.CreditShawn Poynter for The New York Times

The announcement went out that afternoon.

Days later, Mr. Corker’s other dilemma — how aggressively to lace into Mr. Trump — came into clearer view. He had set off on a risky investment, like the rest of the Republican Party, in standing with the president. He had labored furiously to make it work. And it was time, Mr. Corker concluded, to be honest about the dim returns.

“I thought he was extremely patient with President Trump,” said Senator Benjamin L. Cardin, Democrat of Maryland and Mr. Corker’s friend and counterpart on the Foreign Relations Committee, “knowing how quickly he can respond to absurdities.”

Even in the short term, the conflict between Mr. Trump and Mr. Corker carries considerable policy ramifications. Mr. Trump’s decision to disavow the Iran nuclear deal, while declining to unravel it entirely, effectively kicks the issue to Congress, where Mr. Corker’s committee will be central to its fate.

This past week, Mr. Corker released a blueprint aimed at imposing an automatic return of sanctions if Iran was believed to be capable of producing a nuclear weapon within a year, or if it violated other restrictions. He worked with Senator Tom Cotton, Republican of Arkansas and a close administration ally, suggesting that his feud with the president will not undercut cooperation on this score.

And if many Republicans — from fellow senators in his conference to zealous Trump supporters in his own state, which the president carried by more than 25 points last year — have questioned the wisdom of Mr. Corker’s searing public remarks, his hometown has generally greeted the moment with a collective shrug.

“Corker’s right,” said Luke Morin, 76, a Trump voter and lifelong Chattanoogan who said he had socialized occasionally with Mr. Corker in the 1970s. “I’ve kind of turned against Trump now.”

Aides to Mr. Corker, who was vacationing with his family in Florida, did not make him available for an interview.

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In the race’s final weeks, as Mr. Corker struggled to fight off his opponent, he phoned a top adviser, Tom Ingram. “He called me, just almost on a tirade,” Mr. Ingram recalled. “He said: ‘Man, I’m tired of this. I may not be as good-looking, I may not be as smooth-talking, but I know the issues.’”CreditErik S. Lesser for The New York Times

Getting His Hands Dirty

The son of a DuPont engineer, born in South Carolina and delivered to Chattanooga before seventh grade, Mr. Corker often seemed to prefer outdoor labor to classroom fare. He got a job collecting trash and lining off baseball fields, blossoming into a star (if undersize) player himself at his public high school.

He also worked construction, taking quickly to the rhythms and tangibility of a job site and opting for a degree in industrial management at the University of Tennessee. After a stint as a construction superintendent, Mr. Corker put aside $8,000 to start his own company, settling into a career spent amassing a small fortune building shopping centers and later investing in higher-end real estate.

His self-assurance translated to personal affairs. Before their first date, his future wife, Elizabeth, an interior designer, had resisted his calls, insisting she had no time. “Well, look,” he told her, “we know if I have time, you have time.” They were married in 1987.

Seven years later, the political itch first struck, when Mr. Corker unsuccessfully challenged Senator Bill Frist in a Republican primary. After his defeat, the governor at the time, Don Sundquist, another Republican, recruited Mr. Corker to Nashville as the state’s commissioner of finance and administration, a job in which he helped bring the N.F.L.’s Houston Oilers to Tennessee’s capital.

By 2001, after a brief return to business life, Mr. Corker hungered for a more public-facing role: mayor of his hometown, a long-fading manufacturing nexus where dismal air quality once compelled white-collar workers to pack a second shirt for the office to account for airborne soot stains.

Though the opening of an aquarium in 1992 helped jump-start the city’s revitalization, Mr. Corker is credited with seeing the task through, leading a push to overhaul the waterfront, develop a 1,200-acre industrial park that is now home to a Volkswagen assembly plant and helping to fashion Chattanooga — now nicknamed “Gig City” — into a technology hub of the South.

“He can’t stand people operating at non-Corker speed,” said Mr. Compton, who became his chief of staff in the mayor’s office. An editorial cartoonist captured Mr. Corker’s sartorial signature — a tie thrown over his shoulder — with a depiction of a Tasmanian devil-like governing blur with a tie tossed askew.

After four years in the job, a bigger stage beckoned. With Mr. Frist, then the Senate majority leader, retiring, Mr. Corker sought the seat in one of the most-watched races of 2006, facing down Harold E. Ford Jr., a Democratic congressman from Memphis.

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Since arriving in Washington, Mr. Corker has established himself as a dependably conservative but not overly partisan committee leader, eager to hold forth with reporters in the Senate bowels and occasionally grating on colleagues with his windiness and healthy self-regard in a chamber overstuffed with both.CreditDoug Mills/The New York Times

The campaign’s most memorable flourish was a commercial, financed by the Republican National Committee, aimed at Mr. Ford, who is black. The ad included a white woman, bare-shouldered, declaring that she had met Mr. Ford at a “Playboy party” and looking into the camera to say, with a wink, “Harold, call me.”

Mr. Corker strained to distance himself from the spot, which was seen as an unsubtle bid to stir racial stereotypes about black men and white women as Mr. Ford tried to become the first black senator from the South since Reconstruction.

In the race’s final weeks, as Mr. Corker struggled to fight off his opponent, he phoned a top adviser, Tom Ingram.

“He called me, just almost on a tirade,” Mr. Ingram recalled. “He said: ‘Man, I’m tired of this. I may not be as good-looking, I may not be as smooth-talking, but I know the issues.’”

When Mr. Corker was through, Mr. Ingram, who had been scribbling notes on the call, dialed another campaign hand with an update: Mr. Corker had, inadvertently, just written their closing ad for them.

A Reliable Conservative

Since arriving in Washington, Mr. Corker has established himself as a dependably conservative but not overly partisan committee leader, eager to hold forth with reporters in the Senate bowels and occasionally grating on colleagues with his windiness and healthy self-regard in a chamber overstuffed with both. (Mr. Corker, a yoga enthusiast, is among the nimbler senators wending through the halls, though.)

His tenure has not been frictionless. Efforts to tackle entitlement spending and government-secured housing, among other causes, have often produced few immediate gains. While Mr. Trump has accused Mr. Corker of helping to enact President Barack Obama’s nuclear deal with Iran, Mr. Corker opposed the agreement repeatedly. (He did not prevent it from proceeding to a vote on the Senate floor, exposing him to ferocious criticism from some conservatives.)

In recent years, Mr. Corker has also attracted the attention of the F.B.I. and the Securities and Exchange Commission over stock transactions involving a top developer of shopping centers and malls. He has denied any wrongdoing; his team noted that a leader of Mr. Trump’s vice-presidential vetting operation had called the concerns “baseless.”

If his next move remains unclear, once his term expires, friends say he is likely to anchor himself back home — in a mansion guarded on a recent weekday by two non-fearsome dogs on the lawn; at a favorite Italian restaurant, Il Primo, where the hostess has promised “Uncle Bob” a free limoncello pie on his next visit for confronting Mr. Trump; across a downtown that can skew almost bohemian in some patches, with a growing lot of Republicans joining the younger liberal demographic in questioning the president’s abilities.

“There’s nothing here that is accurate,” his Senate chief of staff, Todd Womack, marveled over hamburgers at Main Street Meats, scrolling through @realDonaldTrump’s handiwork on Mr. Corker.

But enough of that, he said, putting the phone away. The waterfront was just down the road.

Mr. Womack returned to his car — with its backside sticker, “Bob Corker for U.S. Senate,” nearly worn away for good — and invited his lunchmate to join him for a Chattanooga drive, to talk a little Bob Corker.

A version of this article appears in print on , on Page A1 of the New York edition with the headline: Senator’s Hometown on Feud With Trump: That’s Just Bob. Order Reprints | Today’s Paper | Subscribe